Two research teams have figured out how to flip a switch in the brain circuits of mice that compels the animals to groom themselves with their paws over and over. The findings may yield new strategies for reducing compulsive behavior such as repetitive hand washing in humans who have obsessive-compulsive disorder and other diseases marked by the trait such as autism and Tourette syndrome.

In one set of experiments, researchers stimulated a particular neural pathway, generating repetitive, exc...